Re: file-backed cgd backup question

[Greg Troxel <gdt%lexort.com@localhost>]

mlelstv%serpens.de@localhost (Michael van Elst) writes:

> vnd has an optimization where the backing file isn't touched, but
> the underlying device is accessed directly. Then file cache and
> device aren't in sync and a backup program reading the file might
> read stale data. vnd should probably update the file when
> unconfiguring, but so far it does not.
>
> The optimization is disabled under some conditions and explicitely
> if you use 'vnconfig -i'. Then all operations are done by file
> I/O and the timestamps of the backing file are maintained.
> The extra caching of course affects performance.

I dimly knew this, but keep forgetting.  Reading vndconfig(8), it does
not explain that the normal path leads to incorrect behavior (stale
reads from file cache even after closing the vnd, mtime).  This
optimization is sufficiently dangerous and not expected that it needs to
be documented clearly and loudly.  I just added a note to the man page.